The present invention relates to a low-frequency (LF) power amplifier and to a preferred use of this amplifier.
The LF power amplifiers for high output powers of conventional types are nearly all built in accordance with the same basic circuit. They contain a negative feedback B-class amplifier having at least two amplifier tubes and an output transformer. The LF signal to be amplified is fed to the grid of the tubes, whose anodes are connected with the terminals of the primary winding of the output transformer. One terminal of the secondary winding of the output transformer is connected via a capacitor to ground, and the other terminal is connected to the load. The feed voltage for the two LF amplifier tubes is fed via a center tap of the primary winding of the output transformer.
LF power amplifiers of the type described can be adapted for very different uses and designed for use at correspondingly different powers, for instance for the excitation of large loudspeakers, in inductive heating devices, in control amplifiers, and as modulation amplifiers for radio transmitters.
All conventional embodiments have certain basic disadvantages for high power output. For the negative-feedback B-class amplifier high-power tubes are necessary, which are expensive and have a comparatively short operating life. These tubes must be operated at high voltage in amplifiers having a high output power, which increases the cost and the overall power consumption of the circuit. The output transformer is a large, heavy component the weight of which may reach six tons in a modulation amplifier for a 300 kW transmitter. Finally, the efficiency of a negative-feedback B-class amplifier controlled by sinusoidal signals and of the load connected to it is dependent upon the degree of modulation; the overall efficiency of the amplifier and load is at best 70%.
The well-known principle of pulse-width modulation offers advantages with respect to efficiency but on the other hand has circuit difficulties inherent in it.
One known solution, disclosed in German Pat. No. 1,218,557, issued Feb. 8, 1963, is characterized by the fact that pulse-width modulated signals are applied in a suitable manner to the control grid of a switch tube, which is at high potential between a high voltage rectifier and the load.
Another solution is disclosed in the journal Funktechnik, vol. 32., No. 13/1977, in which a HF output stage tube is connected in series with a switch tube, which is controlled by pulse-width modulation, the cathode of the HF output-stage tube being grounded with respect to high frequency but oscillating in the rhythm of the oscillation frequency.